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Wireless Analog Signal Transmission


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I know the topic has been brought up quite a few times, but I would like to get your opinion on another way of wireless transmissions:

all topics discussed until now, were about wireless midi transmission (btw: a wirless midi transmission system can be bought now from m-audio; as I've read some time ago!) – but as this requires multiple microcontrollers and serial transmissions, DACs and ADCs and seems like hell of work, time, experience and develpment, I thought about hijacking an A/V wireless transmission device:

that means I want to send the analogue voltage of a sensor directly via 2.4GHz A/V-transmission to a receiver that feeds the voltage into the AIN-pin.

I have some 2.4 GHz Audio/Video transmission devices here; the biggest problem should be to get the required 12V for the sender out of a batterypack. From what I've understood from the docs, a Line-Signal can transmit up to 1.3 V; which should be still sufficient to read out with my sensorizer; maybe a simple OpAmp can increase the signal later on, too...

What do you think of that possibility?

2.4 GHz A/V transmission systems get more and more popular, and so they become quite affordable; Most devices (at least the ones I got from X10) provide up to 4 channels to select, so there could be 4 different AIN-devices...

Does anybody know how many Volts the Line-In of a device can acutally handle in practial situations (will the input of 5V to a Line-In destroy a device?)

Could it be harmful for the Core to put up to 5V Signal directly on the AIN-pin from a different power circuit?

Thanks for your suggestions!



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Hi Michael,

I don´t want to discourage you, but I see problems:

1. "Consumer level" officially is -10 dBV, that´s around 0.3 V. So even if your device can handle 1.3 V, with this voltage there might already be a lot of distortion and I wouldn´t go much higher at all not to damage the device.

If this was the only problem, you could use a voltage divider ("volume pot") to bring the voltage down to e.g. 0...0.5V and use a rail-to-rail OpAmp set to 10x amplification and driven from the 0..5V supply from the core to bring it up again.

2. all wireless systems (also digital ones) use some kind of compander system to increase SNR, so especially with a cheap consumer device you can´t be sure the signal is the same after transmission. It could be more or less compressed afterwards.

3. It´s an audio system, this usually means that the input will be DC-decoupled. So for values from a sensor that change slowly or even not at all for some time, the output will probably go down to zero.

All this makes it really difficult. Esp. the DC issue can possibly only be solved by e.g. AM or FM modulating some kind of carrier frequency and decoding it again afterwards. It would probably be easier and provide more reliable values if you´d try to develop a way to transmit the values digitally.

Here´s one suggestion:  If speed is not too important you could e.g. make up a coding scheme where a PIC sends short 5kHz pulses representing the bits. e.g.

5 ms Pause for "block separation", start bit, two bits sensor number for 4 sensors, 7 bits data value, stop bit. The bits being 2 ms (on/off state) and 1 ms gap inbetween.

With these times, you´d need around 35 ms to transmit one value. At 5kHz, 1ms is 5 periods, so this might be a good value to detect the bits reliably. You can try using 10kHz and shorter times to increase speed depending on signal quality and on how much processing time is left.

To get clean TTL pulses to decode, have a look here: http://www.picbasic.org/articles/ultrasonic/ultrasonic_experiments.pdf. This is a very good basic article about ultrasonic range finding. I used the described cirquit for my experiments and it works really well. In the receiver cirquit you find an analog comparator that is used to generate only high/low pulses to detect with a µC. You can leave out the OpAmp stage, only the comparator is useful to be independent of signal level.

I´d say, this way using your existing device might be possible. But if it´s not only sporting spirit to do it Yourself and you value your spare time with at least 1-2 Euros/hour, it might be cheaper and provide a better (faster) solution to simply buy the M-Audio system.


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Hi Seppoman!

many thanks for your thoughts! :)

I didn't know about the consumer-level and the DC-decoupling; I see now the difficulties that this method most probably implies.

The things get even more weired in my head, because I actually try to get my skin-restistance based Kontaktstation wireless, and to extend it to four or more persons. So I thought it would be easy equipping two persons with plus poles and the other two with minus poles and a wireless transmitted signal, without having the need to build four microcontrolled senders (which would annoy me a bit) :-\

Maybe a possible workaround for the DC-decoupling would be to "encode" the signal in a simple sine-wave and transmit that to the core-station, there decoding (and interpolating) the wave with an OpAmp? ...that would widen the possibility of the kind of transmission; I'm thinking about a bunch of very cheap FM-transmitters for MP3-players I bought recently from ebay  ;D (I know there should be a lot of noise, but maybe some interesting music comes out of it ;) )

However, I think I will experiment a bit; it seems to me, that holds a bunch of fresh ideas, even if I don't reach the aim I got in mind for it now :)

...and reminds me of the light-phone I found in a funny book of old technical inventions: this is a device that transmits sound with some kind of pre-industrial laser (sound -> membrane as microphone -> light from the sun, redirected through a lens, vibrates because of the membrane -> transmitted to receiver as light-beam -> lens-collecting the light -> photoresistive material produces electricity -> forced vibration of receiver-membrane!) :D ...I love this kind of stuff!

However, I'll be back with some testing-reports...

Thanks alot for your suggestions!



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